The scandal at Abu Ghraib prison was first exposed not by a digital photograph but by a letter. In December 2003, a woman prisoner inside the jail west of Baghdad managed to smuggle out a note. Its contents were so shocking that, at first, Amal Kadham Swadi and the other Iraqi women lawyers who had been trying to gain access to the US jail found them hard to believe.

The note claimed that US guards had been raping women detainees, who were, and are, in a small minority at Abu Ghraib. Several of the women were now pregnant, it added. The women had been forced to strip naked in front of men, it said. The note urged the Iraqi resistance to bomb the jail to spare the women further shame.

Late last year, Swadi, one of seven female lawyers now representing women detainees in Abu Ghraib, began to piece together a picture of systemic abuse and torture perpetrated by US guards against Iraqi women held in detention without charge. This was not only true of Abu Ghraib, she discovered, but was, as she put it, "happening all across Iraq".

In November last year, Swadi visited a woman detainee at a US military base at al-Kharkh, a former police compound in Baghdad. "She was the only woman who would talk about her case. She was crying. She told us she had been raped," Swadi says. "Several American soldiers had raped her. She had tried to fight them off and they had hurt her arm. She showed us the stitches. She told us, 'We have daughters and husbands. For God's sake don't tell anyone about this.'"

Astonishingly, the secret inquiry launched by the US military in January, headed by Major General Antonio Taguba, has confirmed that the letter smuggled out of Abu Ghraib by a woman known only as "Noor" was entirely and devastatingly accurate. While most of the focus since the scandal broke three weeks ago has been on the abuse of men, and on their sexual humilation in front of US women soldiers, there is now incontrovertible proof that women detainees - who form a small but unknown proportion of the 40,000 people in US custody since last year's invasion - have also been abused. Nobody appears to know how many. But among the 1,800 digital photographs taken by US guards inside Abu Ghraib there are, according to Taguba's report, images of a US military policeman "having sex" with an Iraqi woman.

Taguba discovered that guards have also videotaped and photographed naked female detainees. The Bush administration has refused to release other photographs of Iraqi women forced at gunpoint to bare their breasts (although it has shown them to Congress) - ostensibly to prevent attacks on US soldiers in Iraq, but in reality, one suspects, to prevent further domestic embarrassment.

Earlier this month it emerged that an Iraqi woman in her 70s had been harnessed and ridden like a donkey at Abu Ghraib and another coalition detention centre after being arrested last July. Labour MP Ann Clwyd, who investigated the case and found it to be true, said, "She was held for about six weeks without charge. During that time she was insulted and told she was a donkey."

In Iraq, the existence of photographs of women detainees being abused has provoked revulsion and outrage, but little surprise. Some of the women involved may since have disappeared, according to human rights activists. Professor Huda Shaker al-Nuaimi, a political scientist at Baghdad University who is researching the subject for Amnesty International, says she thinks "Noor" is now dead. "We believe she was raped and that she was pregnant by a US guard. After her release from Abu Ghraib, I went to her house. The neighbours said her family had moved away. I believe she has been killed."

Honour killings are not unusual in Islamic society, where rape is often equated with shame and where the stigma of being raped by an American soldier would, according to one Islamic cleric, be "unbearable". The prospects for rape victims in Iraq are grave; it is hardly surprising that no women have so far come forward to talk about their experiences in US-run jails where abuse was rife until early January.

One of the most depressing aspects of the saga is that, unaccountably, the US military continues to hold five women in solitary confinement at Abu Ghraib, in cells 2.5m (8ft) long by 1.5m (5ft) wide. Last week, the military escorted a small group of journalists around the camp, where hundreds of relatives gather every day in a dusty car park in the hope of news.

The prison is protected by guard towers, an outer fence topped with razor wire, and blast walls. Inside, more than 3,000 Iraqi men are kept in vast open courtyards, in communal brown tents exposed to dust and sun. (Last month, nearly 30 detainees were killed in two separate mortar attacks on the prison; about a dozen survivors are still in the hospital wing, shackled to their beds with leather belts.) As our bus pulled up, the men ran towards the razor wire. They unfurled banners and T-shirts that read: "Why are we here?" "When are you going to do something about this scandal?" "We cannot talk freely."

The women, however, are kept in another part of the prison, cellblock 1A, together with 19 "high-value" male detainees. It is inside this olive-painted block, which leads into a courtyard of shimmering green saysaban trees and pink flowering shrubs, that the notorious photographs of US troops humiliating Iraqi prisoners were taken, many of them on the same day, November 8 2003. A wooden interrogation shed is a short stroll away. As we arrived at the cellblock, the women shouted to us through the bars. An Iraqi journalist tried to talk to them; a female US soldier interrupted and pushed him away. The windows of the women's cells have been boarded up; birds nest in the outside drainpipe. Captain Dave Quantock, now in charge of prisoner detention at Abu Ghraib, confirmed that the women prisoners are in solitary confinement for 23 hours a day. They have no entertainment; they do have a Koran.

Since the scandal first emerged there is general agreement that conditions at Abu Ghraib have improved. A new, superior catering company now provides the inmates' food, and all the guards involved in the original allegations of abuse have left.

Nevertheless, there remain extremely troubling questions as to why these women came to be here. Like other Iraqi prisoners, all five are classified as "security detainees" - a term invented by the Bush administration to justify the indefinite detention of prisoners without charge or legal access, as part of the war on terror. US military officials will only say that they are suspected of "anti-coalition activities".

Two of the women are the wives of high-ranking and absconding Ba'ath party members; two are accused of financing the resistance; and one allegedly had a relationship with the former head of Iraq's secret police, the Mukhabarat. The women, in their 40s and 50s, come from Kirkuk and Baghdad; none has seen their families or children since their arrest earlier this year.

According to Swadi, who managed to visit Abu Ghraib in late March, the allegations against the women are "absurd". "One of them is supposed to be the mistress of the former director of the Mukhabarat. In fact, she's a widow who used to own a small shop. She also worked as a taxi driver, ferrying children to and from kindergarten. If she really had a relationship with the director of the Mukhabarat, she would scarcely be running a kiosk. These are baseless charges," she adds angrily. "She is the only person who can provide for her children."

The women appear to have been arrested in violation of international law - not because of anything they have done, but merely because of who they are married to, and their potential intelligence value. US officials have previously acknowledged detaining Iraqi women in the hope of convincing male relatives to provide information; when US soldiers raid a house and fail to find a male suspect, they will frequently take away his wife or daughter instead.

The International Committee of the Red Cross, whose devastating report on human rights abuses of Iraqi prisoners was delivered to the government in February but failed to ring alarm bells, says the problem lies with the system. "It is an absence of judicial guarantees," says Nada Doumani, spokesperson for the ICRC. "The system is not fair, precise or properly defined."

During her visit to Abu Ghraib in March, one of the prisoners told Swadi that she had been forced to undress in front of US soldiers. "The Iraqi translator turned his head in embarrassment," she said. The release of detainees, meanwhile, appears to be entirely arbitrary: three weeks ago one woman prisoner who spoke fluent English and who had been telling her guards that she would sue them was suddenly released. "They got fed up with her," another lawyer, Amal Alrawi, says.

Last Friday, about 300 male prisoners were freed from Abu Ghraib, the first detainees to be released since the abuse scandal first broke. A further 475 are due to be released tomorrow, although it is not clear if any of the women will be among them. General Geoffery Miller, who is responsible for overhauling US military jails in Iraq, has promised to release 1,800 prisoners across Iraq "within 45 days". Some 2,000 are likely to remain behind bars, he says. Iraqi lawyers and officials aredemanding that the US military hands the prisons over to Iraqi management on June 30, when the coalition transfers limited powers to a UN-appointed caretaker Iraqi government. Last week, Miller said "negotiations" with Iraqi officials were ongoing.

Relatives who gathered outside Abu Ghraib last Friday said it was common knowledge that women had been abused inside the jail. Hamid Abdul Hussein, 40, who was there hoping to see his brother Jabar freed, said former detainees who had returned to their home town of Mamudiya reported that several women had been raped. "We've know this for months," he said. "We also heard that some women committed suicide."

While the abuse may have stopped, the US military appears to have learned nothing from the experience. Swadi says that when she last tried to visit the women at Abu Ghraib, "The US guards refused to let us in. When we complained, they threatened to arrest us."